Method of and apparatus for the milling of granular materials

ABSTRACT

In a fluid-energy or jet-milling system in which a gas stream entrains a granular or particulate material in a jet against a surface to further comminute or pulverize the milled material, the operating or carrier gas is compressed prior to introduction into the mill, expands as a jet in the mill chamber and is separated from the comminuted solid matter thereafter. The gas stream is cooled prior to introduction into the fluid-energy mill and subsequent to compression and the material to be milled is precooled by the spent carrier gas.

United States Patent 11 1 Weishaupt et a1. July 29, 1975 [54] METHOD OFAND APPARATUS FOR THE 3,614,001 10 1971 Beike 241 23 MILLING OF GRANULARMATERIALS 3,633,830 H1972 Oberprillezr 241/18 3,658,259 4/1972Ledergerber 241/5 [75] Inventors: Josef Weishaupt, Pullach; Jakob3,713,592 1/1973 Beike 241/17 Oberpriller, Baierbrunn, both of G ermanyPrimary ExaminerGranvi11e Y. Custer, Jr. [73] Assignee: LindeAktiengesellschaft, Attorney, Agent, or Firm-Karl F. Ross; HerbertWiesbaden, Germany Dubno 221 Filed: Feb. 7, 1974 [211 App]. No.: 440,446[57] ABSTRACT Related Application Data In a fluid-energy or jet-millingsystem in which a gas 0nlinuati0rl 268,134, June 1972, stream entrains agranular or particulate material in a abafldonedjet against a surface tofurther comminute or pulverize the milled material, the operating orcarrier gas is Forelgn Appllcatlon y Data compressed prioritointroduction into the mill, ex-

July 2, 1971 Germany 2133019 pands as a jet in the mill chamber and isseparated from the comminuted solid matter thereafter. The gas [52] US.Cl. 241/5; 241/18; 241/23 stream is cooled prior to introduction intothe fluid- [51] Int. Cl. B02c 19/06 energy mill and subsequent tocompression and the [58] Field of Search 241/5, 17, 18, 23 material tobe milled is precoolled by the spent carrier gas. [56] References CitedUNITED STATES PATENTS 1 Claim 2 Drawing Flgures 2,836,368 5/1958 McCoy241/17 METHOD OF AND APPARATUS FOR THE MILLING OF GRANULAR MATERIALSThis is a continuation of application Ser. No. 268,134, filed June 30,1972, now abandoned.

FIELD OF THE INVENTION Our present invention relates to fluid-energy orjet milling and, more particularly, to a method of and an apparatus forthe comminution of materials by fluidenergy or jet-milling principles.

BACKGROUND OF THE INVENTION Fluid-energy or jet mills generally make useof a plurality of orifices or nozzles directed toward a wall of amilling chamber or trained on each other and are designed to entraingranular or particulate materials against this wall in a grinding andshearing action to pulverize, abrade and round off the granularmaterials. Some fluid-energy or jet mills comprise a circular or otherround grinding chamber to which the fluid is admitted in finehigh-velocity streams at an angle around a portion of, or all of, theperiphery of the grinding chamber. In another class of fluid-energy orjet mill, two or more fluid streams convey the particles into a chamberin which the two streams impact upon each other and upon any of thechamber surfaces. Whether the particles are conveyed with the jet or areintroduced into the path of jets traversing the grinding chamber, thereis a high-energy release and a high order of turbulence which causes theparticles to grind upon themselves and to break down by impact, abrasionand friction. Most often, such mills also carry out a classifyingoperation, i.e., the emerging gas carries off the micronized powder.

In fluid-energy or jet mills it has been proposed to use a drivingfluid, i.e., a gas at the grinding temperature,

which is compressed prior to introduction into the jet or fluid-energymill chamber, which expands in this chamber and which downstream of thechamber is separated from the resulting particles, i.e., micronizedpowder.

For fine milling and for the finest comminution of granules or coarseparticulate substances, such fluidenergy or jet mills have the advantagethat no mechanical rotating or oscillating mill parts are required. Thisnot only reduces the cost of the apparatus and the tendency to breakdown but ensures comminution with a minimum of contamination by foreignmatter from the surfaces of a milling chamber.

In counterflow fluid-energy or jet mills, i.e., mills of the characterdescribed whereby two particleentraining streams are directed againstone another, the carrying gas may be derived from steel bottles orflasks and is permitted to flow through two venturi nozzles in parallelinto the fluid-energy or jet mill chamber, the reduced pressuredeveloped in the venturi nozzle together with increased velocity causesthe granular or coarse-particle material to be entrained with high speedalong a trajectory counter to the trajectory of the particles of theother stream. When two particle streams moving at high velocity and withhigh kinetic energy in opposite directions collide, the impact releasesthis kinetic energy in the form of energy of breakdown whereby thestructure of the granules is altered or destroyed. Glancing collisionshave a similar effect and are also valuable because they provide amutual abrasion and rounding of the particles.

Fluid-energy or jet milling occurs most readily in the region of theperiphery of the milling chamber toward which the particles are carriedby the centrifugal forces which induce a rotary movement of theparticles within the chamber. The comminuted or subdivided particleswhich have already been reduced to the desired state of fineness havethe smallest mass and tend to collect in the central regions of themilling chamber from which they are removed and may be subjected tofurther classification in addition to being separated from the expandedgas stream leaving the mill.

The gas/solid separator may be any of the devices known in the art andcan include sedimentation chambers, screens, cyclones, and the like. Itwill be apparent that the kinetic energy of the particles of thecounterflow fluid-energy or jet mill appears in large measure as heatwhich may be detrimental for certain materials so that reducedthroughputs may be necessary to prevent such overheating. This solutionhas the disadvantage, especially in the case of low throughputs, thatthe energy which is supplied per unit weight or volume of certainmaterials is excessive while with other materials having relativelysignificant elasticity and viscosity is insufficient so thatsatisfactory milling does not occur. In short, fluid-energy and jetmilling of synthetic resins and other substances with a high velocityand pronounced plastic deformability cannot be satisfactorily conductedby such apparatus or techniques.

OBJECTS OF THE INVENTION It is an important object of the presentinvention to provide an improved method of comminuting materials,especially materials of high plastic deformability, especially syntheticresins.

Another object of the invention is to provide an improved method ofmilling difficult-to-mill materials whereby lower-energy requirementsfor a given throughput or higher throughputs for a given energy can beattained.

Still another object of the invention is to provide an improvedapparatus, and a method of operating same, which is especially effectivefor materials which have high viscosities and which have not hithertobeen adequately or satisfactorily milled by fluid-energy or jetmillingtechniques.

SUMMARY OF THE INVENTION These objects and others which will become morereadily apparent hereinafter are attained, in accordance with thepresent invention, with a system which represents an improvement overconventional fluidenergy or jet-milling techniques and involves thecooling of the driving gas or other fluid prior to its expansion in thefluid-energy or jet-milling chamber, but subsequent to its compression.According to a basic principle of the present invention, therefore, acarrier or separating gas is compressed and then cooled, preferably in aheat-exchange relationship with an expanded fluid, prior to introductioninto the fluid-energy or jet mill, is then expanded in the chamber ofthe latter to comminute solid materials entrained by the gas stream orstreams, and is separated from the gas subsequent to milling.

An important advantage of the preliminary cooling step, according to thepresent invention, i.e., the cooling 'of the driving gas prior to itsexpansion into the fluid-energy or jet mill but after compression, isthat it enables the milled material to be brought to low temperaturesand even to cryogenic temperatures, i.e., the temperatures of liquefiedgases such as nitrogen and oxygen. In this way, the material to becomminuted may be embrittled to facilitate pulverization in the fluidenergy or jet mill. In this case, the temperature of the gas is reducedto a point such that the material to be comminuted is no longerplastically or elastically viscous but ruptures readily upon impact witha surface or another particle. In some cases the embrittlement of thematerial itself may produce a fragmentation. In general, embrittlementof the material to be comminuted results in a substantial increase inthe throughput of the apparatus for a given energy and hence representsa substantial increase in efficiency.

According to a more specific feature of the invention, the heatabstracted from the driving gas subsequent to compression and prior toexpansion into the mill chamber is transferred by indirect heat exhangeto the expanded and thereby cooled gases previously used in the millchamber, subsequent to separation of the gases from the solids. In otherwords, the compressed drive-gas stream, before its introduction into thejet mill, is cooled at least in part by heat exchange with the expandedcold drive-gas stream which emerges from the jet mill. The degree ofcold retained by the expanded gas stream is, of course, increased as theheat generated in the jet mill is decreased and vice versa.Consequently, any increase in efficiency is multiplied by enabling thelow-temperature output gases to cool the incoming compressed gases.Since the cooling step increases efflciency not only by embrittlementand by the removal of large quantities of usable heat from the materialsubjected to comminution, but for the other reasons mentioned earlier,substantially less heat is produced by conversion of kinetic energy ofmovement into heat energy of collision or friction so that the maximumcold can be maintained in the expanding gas stream after milling, themill can be maintained at extremely low temperatures in a convenientmanner, and the maximum temperature differential, during heat exchange,between the compressed warm gases and the expanded cold gases isobtained. It should be noted that an increased temperature differentialrepresents an increased heat transfer rate and enables the heatexchanger to be of relatively small dimensions.

A particularly important advantage of the system of the presentinvention is that with a correct or optimal establishment of thedriving-gas quantity, degree of compression, degree of expansion, flowrate, etc., it is possible to obtain embrittlement and the advantagesenumerated above with minimum introduction of cold from some foreignsource, the energy losses being supplied in the form of power fordisplacing the fluids. For peak requirements of cold of the drive-gasstream, however, we may make use of an external cold source which mayconstitute a conventional refrigeration plant or may be a source ofliquified gas which can be sprayed directly into the powerfluid lines.The additional cold may also be supplied by indirect heat exchange witha liquified gas. In any case, the external source preferably suppliescold to the drive-gas stream intermittently with the cold pulses havingan on-time" and off-time dimensioned to enable the driving fluid to bebrought to the lowest suitable temperature and yet enable anyestablished temperature in the milling chamber to be held withoutdifficulty.

According to still another feature of the invention, the driving-gasstream is recirculated in a single path including compression, coolingand expansion, thereby reducing energy and gas consumption and allowingthe cooling and milling rate to be controlled with ease. A portion ofthe cold gas stream may be branched from the main stream and may bepassed through the material to be milled before it enters the millingzone, thereby precooling the solids to be comminuted. Advantageouslythis precooling step is carried out in a column with the solids passingin countercurrent (counterflow) to the branched portion of precoolinggas. The precooling can be sufficient to produce the embrittlementmentioned earlier. The partially warm branched gas stream can be thenreleased into the atmosphere or recycled.

Still another feature of the present invention resides in the cooling ofthe driving-gas stream in a plurality of stages, for example includingan initial stage in which it is cooled by heat exchange with theexpanded cold gas stream to a low temperature and a second stage inwhich externally supplied cold brings the gas to still lowertemperatures, e.g., in the cryogenic ranges (lowest temperatures). Theterm cryogenic temperature or cryogenic range is intended to refer totemperatures in a range of the liquefaction temperatures of such gasesas argon and the other inert gases, nitrogen and oxygen, or therebelow.These temperatures lie generally below minus C (about 173K). Themultistage process enables the energy and cold balance and transfer tobe controlled readily and to obtain optimum cooling or embrittlement ofthe milled product.

As already noted, it is in the nature of the fluidenergy or jet millthat some degree of classification of the comminuted product occursduring milling and in the process whereby the driving gas is separatedfrom the solids. This separation can be enhanced by furtherclassification, e.g., by particle size, or a particular particle-sizefraction may be recovered while other fractions are discarded or remainin the mill. Advantageously, water and other solvent vapors releasedfrom the milled material and carried off by the expanded gas stream maybe removed by absorption in columns or the like prior to recirculation.

When the material to be comminuted is a substance which is not to comeinto contact with oil, the compression step may be carries out in anydry-running compressor. When oil contamination is not a problem, anoil-lubricated compressor may be used. According to still anotherfeature of the invention, the compression step may take place remotefrom the installation at which the milling occurs and in that case weprefer to provide steel cylinders or other compressed-gas bottles whichmay be charged elsewhere with the driving gas. The compressed drivinggas is connected via a duct to a heat exchanger, preferably a coiledtube countercurrent heat exchanger, in which the compressed and warmdriving gas and the expanded cold gas removed from the chamber arepassed in countercurrent. The compressed cooled gas can then beconducted via a further duct to the fluid-energy or jet mill and entersthe latter through one or more nozzles.

The operating gas thus expands and subjects the material to becomminuted to the high-energy stream whereby the solid material ispulverized as described conventional dosing device, e.g., a wormor screwadapted to supply the material to be communuted at,

the optimum rate regulated in accordance withthe parameters of theoperating gas. The comminuted mate rial, preferably from a shaft orcolumn having the conveyor worm at its base, can be supplied with abranched portion of the cold gas as previously described. From thefluid-energy or jet mill, the expanded cold gas stream, carryingparticles of the desired fineness, is delivered to any conventionalgas/solid separator in which the milled product is recovered from theexpanded operating gas. The gas/solid separator and the heat exchangerare connected by a further duct so that the expanded cold gas stream canbe directed to the latter for indirect heat exchange with the compressedwarm gas as described. The heat exchanger, in turn, may be connectd withstill another duct leading to the compressor so that the expanded gas,after removal of heat from the compressed gas, is supplied to thecompressor itself and is passed in a recirculating flow.

The external-cooling device, which may be a refrigerating unit, as notedabove, is preferably disposed between the gas/solid separator and theheat exchanger along the cold/gas duct. At least part of the workinggases used in the system is preferably conducted through this unit andthereby cooled. In order to obtain an especially high efficiency andreduce the energy supplied to the system, and also to obtain maximumutilization of space, the heat exchanger, material-feed device,material-cooling column, jet mill and separator are so constructed thatthey form a compact unit which is surrounded by a thermally insulatingshell. In this compact configuration, the feed shaft and cooling columnare provided above the milling chamber and the gas/solid separator isthen located in line therewith so that the gas stream may enter the heatexchanger without entrainment of comminuted material.

DESCRIPTION OF THE DRAWING pact apparatus embodying principles of theinvention.

SPECIFIC DESCRIPTION In FIG. 1 of the drawing, we have shown a flowdiagram of an apparatus embodying the present invention and whichconstitutes a cooled jet mill or fluid-energy mill represented generallyat 1. Theplant comprises a compressor 2, e.g., a dry-running compressorif the ma terial to be milled must remain freeof oil, or anoillubricated compressor if. oil contamination is no problem. Thecompressor 2, which may be of the type described at pages 6 2 6 32vofPERRYS CHEMICAL ENGINEERS HANDBOOK (McGraw-Hill Book Co., New York,1963). preferably should have a capacity such that the emerging gasesare at a pressure of about 8 atmospheres gauge. The duct 3a carries thecompressed warm gas through a heat exchanger 5 and a heat exchanger 4supplied with cold water, e.g., from a cooling tower (line 4a), the warmwater being reconveyed to the tower via line 4b. The heat exchanger 5 ispreferably of the cooled-tube counterflow type wherein the relativelywarm compressed fluid passes through the small crosssection channelafforded by the tube coil 5d while the expanded gas stream flows throughthe large-cross-section shell 5b.

The cooled compressed gas stream is then led via line 3b to thefluid-energy or jet mill 6 which is preferably of the type described atpages 8 42 ff. of PERRYS CHEMICAL ENGINEERS HANDBOOK. The gas expandswithin the mill chamber and projects a plurality of countermoving jetsentraining the particles of material against one another to comminutethe material such that the expanded gas stream recovered at 3c entrainsthe fine particles therewith. The jet mill may be provided with one ormore nozzles as described in the cited publication and is preferably ofthe high-velocity, opposing-jet type.

The material is delivered :from a cooling and feed shaft 7 via a wormconveyor 8 to the mill chamber whereby the particles are comminuted andabraded in accordance with the usual jet-mill principles.

The fine-particle fraction entrained by the gas stream and emerging fromthe opening 9 of the mill is carried along to the gas/solid separator 10(line 30) in which the comminuted or pulverized material is separatedfrom the expanded cold gas stream, the particles being collected in acolumn 10a for further use. The separator 10 may be of any conventionaltype, e.g., a cyclone or sedimentation column (see pages 20 68 ff. ofPER- RYS CHEMICAL ENGINEERS HANDBOOK).

The cold gas, freed from the major portion of the particles in thecyclone and any residual particulate matter in a filter or the like, ispassed through an absorption column 15 via line 3d, the absorptioncolumn being operated in accordance with conventional principles usingcalcium chloride or other dessicants. The expanded cold gas is deliveredvia line 3e to the outer shell of heat exchanger 5 and is passed vialine 3f to the compressor 2 to complete the gas cycle represented atZia-3f.

In the duct 3e which may be provided with a valve 3e so that all or partof the fluid may be passed through level 12a, there is provided acooling arrangement for delivering external cold to the system orabstracting heat therefrom. The external cooling system comprises theline 12a which conducts all or part of the gas traversing line 3e to aliquefied-gas bath l3, e.g., of liquid nitrogen. The gas of theoperating cycle of the mill passes through the tube coil 13a of thistank in which the liquefied gas surrounds the coils and also is providedwith a valve 13b whereby some of the cooling fluid is permitted toexpand and evaporate into a line 12c. The main body of the divertedportion of the operating gas is carried into line 12b from which itpasses via a jet valve 12d into line 32. From line 12c, the evaporatedfluid from the external'cooler 13 can be led to line 12b andtherebyin'troduced into the cooling cycle.

The external cooler also may be provided with a conduit l4 and a valve14a leading a portion of the expanded cold gas to the base of the column7 so that this cold gas passes through the column in counterflow to thedescending material to be comminuted. If desired, this cooling streammay be replaced in whole or in part by a portion of the cooledcompressed gas which may be bypassed from line 3b via a valve 3b and aduct 3b" to the base of the column 7. The gases used to precool thecomminuted material may be vented via a line 14b and a valve 14c or maybe returned to line 3f via the conduit 3f. Preferably the valve 3e ispulsed with an on-time and off-time which is determined by a pulser 3e"at a rate such that the external cooling is supplied intermittently, thepulser comprising an oscillator and coil as described on page 13 ofPULSE GENERA- TORS by Glasoe and Lebacqz (McGraw-Hill: 1948 Theoperation of the system of FIG. 1 will be immediately apparent. Thecompressed gas, which may be at a pressure of 8 atm g. and may benitrogen, is cooled in the water cooler 4 before entering the heatexchanger 5. In the heat exchanger 5, the compressedgas is cooled to acryogenic temperature and enters the jet mill 6 in the usual manner. Aportion of the gas is diverted through the column 7 and expands thereinto precool the material, which may be polyethylene granules, to a lowtemperature, the embritted granules being carried into the mill 6 andtherein comminuted to a particle size of the order of microns. Theemerging gas stream, at a cryogenic temperature, is separated from theparticles and returned to the heat exchanger in which it cools thecompressed gas as described. When the throughput is such that externalcooling is required, a portion of the expanded cold gas from theabsorber is conducted to the liquid nitrogen tank 13 in which it iscooled to a temperature slightly above the boiling point of nitrogen atatmospheric pressure and is returned to the cold gas stream.

In FIG. 2, we have shown a desirable construction of the mill accordingto the invention in which the jet mill 16 is surmounted by a feedandcooling shaft or column 17, the latter being surrounded by a heatexchanger shell 18 and provided along its periphery ahead of the heatexchanger with a particle separator 19. The entire structure issurrounded by a thermally insulating layer 20 and stands on a support21.

The shaft 17 is the upper continuation of the separator l9 and istheinner tube of the exchanger 18. A chamber 22 is formed within theseparator into which the gas from the mill 16 along with the completelycomminuted product blows. The separator 19 surrounding this chamber 22is formed as a screen or grate in order to allow only the gas to passradially outwardly where it flows over the coils of the exchanger 18.The completely milled particles fall in the chamber 22 into passages 23which open into a funnel 24 at the base of the installation. Thencethese particles pass through a rotary airlock feeder 41 into a container25.

A stage approximately 4 meters above the base of the device supports acompressor 26 connected in a refrigerant circuit 27 including areservoir 28 as well as associated equipment 29 (e.g., a compressor asdescribed with respect to FIG. 1). A conduit 30 transfers the compressedgas from the compressor 26 to the coil of the heat exchanger 18 and aconduit 30' conducts the decompressed gas back to the compressor 26.

A conduit 42 leads from the bottom of the heat exchanger coilsdownwardly in the installation to inject a portion of the compressedgases through a radially opening nozzle 43 at the bottom of the mill 16.The jet pulverizer 16 is of the opposed-jet type. The material to bemilled is fed to the pulverizer 16 via a central tube 16a so as to beexposed in the chamber 16b to the opposed jets issuing from nozzles 43and 44. The particles are milled by striking each other and thesufficiently comminuted granules pass up out of the chamber 16b with thespent milling gas through passages 16c terminating in the chamber 22.Another conduit 44 and nozzle 45 opening directly opposite the nozzle 43injects gas also into the mill 16 in order to comminute particles in themanner described above.

The refrigerant circuit 27 is connected via a heat exchznger 31 similarto the exchangerS of FIG. 1 to the circuit 30 in order to maintain asufficiently low temperature in the device. A conduit 32 leads from thepath 30 downstream of the exchanger 31 to the base of the precooling andstorage shaft 17. Thus the particles in this hopper 17 flowcounter-current to the extremely cold gases which are recovered at thetop via a conduit 34 and either vented to the atmosphere through a valve33 or feed back to the circuit 30 through a valve 35 and dust filter 36.

Another rotary airlock feeder 40 is provided at the base of the shaft 17to prevent gas loss from the mill 16. Yet another such feeder 39 isprovided at the top of the shaft 17 so that particles may be fed theretofrom a hopper 38 without pressure or material loss.

We claim:

1. A method of comminuting pieces of a material comprising the steps of:

a. confining a supply of said material with a wall and cooling saidsupply of material by heat exchange with a gas through said wall;

b. compressing said gas after psssing it in heat exchange with saidsupply of said material through said wall;

c. cooling the gas compressed in step (b) by passing it in heat exchangewith a recirculated external coolant to produce a cold fluid;

d. expanding a portion of the cold fluid produced in step (c) andpassing the expanded portion of said fluid through said supply in directcontact with said material whereby water and other vapor is entrainedwith said portion;

e. passing the remainder of the cold fluid produced 1 in step (c) inindirect heat exchange with the gas cooling said supply of material instep (a);

f. metering said material into a jet mill after said material iscontacted with said portion of said cold I fluid in step (d);

g. pulverizing said material to a powder in said jet mill by entrainmentof said material in the fluid passed in indirect heat exchange with saidgas in step (e); f

h. separating the fluid used to pulverize said material instep (g) fromthe resulting powder; and

i. feeding the fluid separated in step (h) as the gas employed in step(a).

1. A METHOD OF COMMINUTING PIECES OF A MATERIAL COMPRISING THE STEPS OF:A. CONFINING A SUPPLY OF SAID MATERIAL WITH A WALL AND COOLING SAIDSUPPLY OF MATERIAL BY HEAT EXCHANGE WITH A GAS THROUGH SAID WALL, B.COMPRESSING SAID GAS AFTER PASSING IT IN HEAT EXCHANGE WITH SAID SUPPLYOF SAID MATERIAL THROUGH SAID WALL C. COOLING THE GAS COMPRESSED IN STEP(B) BY PASSING IT IN HEAT EXCHANGE WITH A RECIRCULATED EXTERNAL COOLANTTO PRODUCE A COLD FLUID, D. EXPANDING A PORTION OF THE COLD FLUIDPRODUCED IN STEP (C) AND PASSING THE EXPANDED PORTION OF SAID FLUIDTHROUGH SAID SUPPLY IN DIRECT CONTACT WITH SAID MATERIAL WHEREBY WATERAND OTHER VAPOR IS ENTRAINED WITH SAID PORTION, E. PASSING THE REMAINDEROF THE COLD FLUID PRODUCED IN STEP (C) IN INDIRECT HEAT EXCHANGE WITHTHE GAS COOLING SAID SUPPLY OF MATERIAL IN STEP (A), F. METERING SAIDMATERIAL INTO A JET MILL AFTER SAID MATERIAL IS CONTACTED WITH SAIDPORTION OF SAID COLD FLUID IN STEP (D), G. PULVERIZING SAID MATERIAL TOA POWDER IN SAID JET MILL BY ENTRAINMENT OF SAID MATERIAL IN THE FLUIDPASSED IN INDIRECT HEAT EXCHANGE WITH SAID GAS IN STEP (E) H. SEPARATINGTHE FLUID USED TO PULVERIZE SAID MATERIAL IN STEP (G) FROM THE RESULTINGPOWDER AND I. FEEDING THE FLUID SEPARATED IN STEP (H) AS THE GASEMPLOYED IN STEP (A).